This invention relates to an electropneumatic transducer in which a moving coil is precisely positioned over a valve opening by a unique adjustment spring mechanism.
Electropneumatic transducers are commonly used to transduce electrical current to pneumatic pressure, especially when a high degree of linearity between the two is required. A voice coil, such as the electromagnetic coils used in speaker systems, has been found particularly useful for the electrically responsive element in these transducing devices.
The basic operating principle of the voice coil is well known. Current is supplied to a coil suspended in a radial magnetic field created by a concentric permanent magnet circuit. The current interacts with the magnetic field by impinging at right angles on the turns of the coil. This interaction creates an axial force on the coil, causing the coil to move in the desired direction. The permanent magnet is prevented from moving by being fixed to the frame of the transducer.
In an electropneumatic transducer, this motivating force on the coil can be applied to the outer surface of a precisely manufactured ball sitting in the circular throat of a pneumatic leakport. The electromotive force on the coil, applied to one surface of the ball, balances the force of the airstream from the leakport against the opposing surface of the ball. The more electric current applied to the electromagnet, the greater the force applied against the ball. Conversely, a reduction of electric current applied to the coil lessens the force applied against the ball.
The pneumatic pressure within the system can be regulated, therefore, by varying the pressure applied by the coil to the ball. An increase of current applied to the electromagnet produces more back pressure in the pneumatic system. The pneumatic pressure is similarly reduced by reducing the current provided to the electromagnet. The relationship between current and air pressure is very linear, enabling the air pressure to be precisely controlled by balancing the pressure applied to the opposing inner and outer surfaces of the ball. Other kinds of pressure regulators can, of course, make use of a moving coil system.
As a practical consideration, the mechanism suspending the coil in position within the magnetic field must exactly position the coil over the pneumatic leakport. Exact positioning, including an offset adjustment, is needed to compensate for inevitable manufacturing tolerances and other factors that affect calibration. It is also crucial that the parts of the offset adjusting mechanism, once adjusted, do not slip or shift from their positions. Since the nominal operating travel of the coil in a typical electropneumatic transducer can be as little as approximately 0.001 inches, the slightest shift between the component parts can cause a noticeable unwanted pressure change in the pneumatic system. The offset adjustment mechanism must not allow any such slippage or sliding.
Conventional offset adjustment mechanisms are usually complicated to use, requiring adjustment of a number of parts. In addition, offset adjustments often rely on frictional contact between the parts, making them susceptible to shifting or slipping. Also, the adjustment and operational components in some prior art transducers do not always move freely, and contact between them produces unwanted hysteresis. The general construction of electropneumatic transducers having offset adjusting mechanisms subject to the above-described and other problems is shown in such patents as U.S. Pat. Nos. 3,861,411, to Mitchell et al; 4,512,357, to Earl; 3,768,772, to Vischulis; and 3,004,546, to Robins.